1. Field of the Invention
The present invention relates to an apparatus which includes a plurality of component holders one of which is selectively used for transferring an electronic component and rotates the selected component holder about an axis line thereof for rotating the electronic component, and particularly relates to the improvements of a device which selects one of the component holders and rotates the selected component holder.
2. Related Art Statement
There is known an electronic component ("EC") transferring apparatus which includes a plurality of component holders. For example, the EC transferring apparatus is employed in an EC mounting system which mounts ECs on a workpiece such as a printed circuit ("PC") board. Recently, the number of the kinds of ECs which are mounted on a workpiece tends to increase. Hence, the EC mounting system is provided with a plurality of component holders having different dimensions and shapes corresponding to different kinds of ECs, and those component holders are selectively used for mounting the different kinds of ECs.
Meanwhile, in some cases, it is required that the selected component holder be rotated about an axis line thereof. For example, in the case where an image of the EC held by the component holder is taken to detect an error of holding of the EC and an error of holding of the EC about an axis line of the EC (hereinafter, referred to as the "angular error" of EC) is detected, it is required that the EC be rotated about the axis line thereof to correct the angular error. In addition, in the case where an EC which has an angular position different from an angular position at which the EC is supplied from an EC supply device to the EC mounting system is mounted on a workpiece, it is required that the EC be rotated about the axis line thereof to change the original or first angular position to the second angular position. In each case, the component holder needs to be rotated. In the case where the EC mounting system includes a plurality of component holders, at least one of the component holders which is being positioned at an operating position where the said one component holder can receive an EC from the EC supply device and hold the EC, must be rotatable about the axis line thereof.
Japanese Patent Application laid open for inspection purposes under Publication No. 2-69998 discloses an EC mounting system which employs a single electric motor as a common drive source for not only selecting one of a plurality of component holders but also rotating one component holder being positioned at an operating position. More specifically described, the disclosed EC mounting system includes a rotary table which is rotatable about a vertical axis line, and a plurality of nozzle holders which are supported by the rotary table such that each nozzle holder is rotatable about a vertical axis line thereof and which are provided along a circle whose center is located on the axis line of the rotary table and are equiangularly spaced from each other about the same axis line. Each nozzle holder supports a plurality of component suction nozzles which have respective vertical axis lines and which are provided along a circle whose center is located on the axis line of the each nozzle holder and are equiangularly spaced from each other about the same axis line. While the rotary table is intermittently rotated, each of the nozzle holders is sequentially stopped at each of a plurality of working positions including an EC sucking position and an EC mounting position where each nozzle holder sucks an EC and mounts the EC. When each of the nozzle holders is rotated about an axis line thereof by the electric motor mounted on the rotary table, one of the suction nozzles is selected and moved to the operating position. When one nozzle holder is rotated about the axis line thereof that is parallel to the axis line of each suction nozzle, each suction nozzle is revolved around the axis line of the nozzle holder and simultaneously is rotated about the axis line of the each suction nozzle. Thus, an angular error of an EC held by each suction nozzle may be corrected. However, when this correction is effected, the EC held by the suction nozzle is revolved around the axis line of the nozzle holder, that is, the position of the center of the EC is changed. Hence, a PC board as a workpiece is moved so as to eliminate the positional change, before the EC is mounted thereon.
In the EC transferring apparatus employed in the above EC mounting system, the electric motor as the drive source is provided on the rotary table, and the selection and rotation of each suction nozzle are effected by rotating the nozzle holder while the rotary table is rotated. Thus, a long time can be used for the nozzle selection and rotation, and accordingly each suction nozzle can be rotated at a small angular acceleration. Therefore, the angular error of the EC is corrected while the out-of-position movement of the EC is prevented. In addition, the angular pitch at which the nozzle holder is rotated to select and position each of the suction nozzles at the operating position may be increased, or the nozzle holder can be rotated by two or more angular pitches at one time.
Moreover, the above EC transferring apparatus enjoys a simpler and lighter construction and a lower production cost than an apparatus which utilizes different electric motors, each mounted on a rotary table, for selecting and rotating an EC, respectively. In addition, since each nozzle unit including each nozzle holder, the electric motor, and a rotation transmitting device which transmits the rotation of the motor to the each nozzle holder enjoys a small mass, the vibration of the rotary table and the nozzle units which is produced when the rotary table is intermittently stopped at the working positions is reduced.
However, in the EC transferring apparatus disclosed in the above-indicated publication, the EC is rotated by revolving the suction nozzle holding the EC around the nozzle holder, so that an centrifugal force is exerted to the EC and an out-of-position movement of the EC tends to take place. Before the EC is rotated, a positional error of the EC is detected and corrected. However, since the above out-of-position movement of the EC takes place after the positional error of the EC is corrected, the out-of-position movement of the EC cannot be corrected.
Moreover, the axis line of each suction nozzle is offset by a certain distance from the axis line of the nozzle holder. Accordingly, if the nozzle holder is rotated by an inaccurate angle for rotating the EC, the position of the axis line of the nozzle holder is subject to an error corresponding to the product of the above offset distance and the angular error of the nozzle holder. This error is maintained as it is when the EC is mounted at a predetermined EC placing position on the PC board.
As described above, when the angular error of the EC is corrected, the position of the EC in the horizontal plane (i.e., X-axis and Y-axis direction positions) changes. Accordingly, the PC board is moved so as to eliminate the positional change of the EC. Thus, a longer time may be needed for moving the PC board from the prior EC placing position to the current position. In consideration of this, a longer cycle time at which ECs are cyclically mounted is employed, and the efficiency of mounting of ECs is lowered.